1. Field of the Invention
This invention is in the field of internally shielded X-ray tubes wherein a high density attenuating means is located in proximity to the path of electrons emanating from the electron gun of the tube and also at or near the target area thereby reducing the need for large amounts of external shielding. Problems inherent in secondary X-ray emission from such high density attenuating means are reduced by providing a lower density shielding about the high density attenuating means.
2. Description of the Prior Art
In any X-ray tube, X-rays are produced by accelerating electrons to a high velocity by means of an electrostatic field and then suddenly stopping them by collision with a solid target interposed in their path. The X-rays which result radiate in all directions from the spot from the target where the collisions take place. The X-rays are due to the mutual interaction of the fast moving electrons with the electrons and the positively charged nuclei which constitute the atoms of a target.
The first high vacuum X-ray tube used a hot tungsten filament cathode and a solid tungsten target. This hot cathode, high vacuum type of X-ray tube permitted stable and reproducible operation with relatively high voltages and large masses of metals. The vacuum was sufficiently good so that positive ions did not play either an essential or a harmful role in the tube operation.
Notwithstanding the wide acceptance of such high vacuum X-ray tubes in recent years, these tubes still have stray radiation problems resulting from high energy photons which are generated at the target and which do not find their way out of the envelope through the window. Consequently, it has become common practice to use large amounts of external shielding to attenuate these X-ray photons outside of the evacuated envelope. The necessity of such shielding significantly increases the cost and the bulk of the X-ray tube.
Some efforts have been made to employ anode shields or hoods consisting of a tubular type member which surrounds the target in an attempt to cut off any stray X-ray beams from particularly low voltage tubes. Such a hooded tube is described in Atlee U.S. Pat. No. 2,754,514.
The advantages of the hooded structure are somewhat reduced by the fact that secondary electrons in an operating tube intercept the anode hood and generate undesirable X-rays. When the hood is made of a high density material, these X-rays are more energetic and require much more external shielding.